There are a number of known techniques for packaging die, including Ball Grid Array (BGA), Land Grid Array (LGA) and Quad Flat No-Lead (QFN). BGA and LGA packages use multi-layer organic substrates which are expensive because the manufacturing process is complex and the raw materials costly. The substrates are made from layers of metal with meshed fibre-glass reinforced epoxy core or dielectric. QFN packages are fabricated by attaching the die to a metal leadframe and forming electrical connections between the leadframe and pads on the die using wirebonds. The whole structure is then over molded (or ‘over moulded’) leaving one face of the leadframe exposed on the base of the QFN package. BGA packages include an array of solder balls which are used to mount the package onto a substrate, such as a printed circuit board (PCB). LGA and QFN packages do not comprise solder balls and are instead mounted on a substrate using solder paste which may be stencil printed onto the substrate.
For pin counts below around 100, QFN packages are lower cost than BGA and LGA packages because of the substitution of a low cost lead frame for the expensive substrate. However, for pin-counts in excess of around 40, QFN packages are larger than a corresponding BGA/LGA package. Additionally, with high pin-counts, both QFN and LGA have a poor yield for the package mounting process because of warpage during solder reflow. The problem is less for BGAs because the molten solder balls can accommodate a degree of warpage.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known package substrates and packaging techniques.